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Monday, January 13, 2014

Shooting strings

String theory, once hailed as theory of everything, now struggles to demonstrate its use for anything at all.

Most string theorists today, if not working for banks, study the gauge-gravity correspondence. This celebrated idea, arguably one of the most interesting findings in string theory, relates a strongly coupled field theory in flat space to weakly coupled gravity in a higher dimensional space. These higher-dimensional spaces do not resemble our universe, so the interesting applications of the gauge-gravity correspondence are analytical calculations in strongly coupled field theory. Notoriously difficult problems of the field theory can become manageable by reformulating them in the language of gravity.

The most widely promoted use of this gauge-gravity correspondence has been the quark gluon plasma, which is produced in highly energetic collisions of heavy ions, previously at RHIC and now at the LHC. There has been a lot of hype about the low viscosity that was analytically found using the gauge-gravity correspondence and that fit well with observations. But heavy ion physics isn’t just viscosity. There are many other observables that a good model must be able to explain.

One of these observables is the energy loss that elementary particles experience when they travel through the plasma. Just by chance it can happen that a particle pair is created but only one of the two particles travels through the plasma and loses energy. The primary particles are unstable and eventually decay to form stable hadrons. By measuring and summing up the momentum of the decay products one can infer the energy loss that happened in the plasma.

We previouslysaw that the gauge-gravity correspondence seems to work well for the RHIC data, but misses the mark when the more recent LHC is also taken into account. The prediction is far outside the error margin of the data, both in terms of magnitude and in terms of slope. The gauge-gravity correspondence predicts too much energy loss. I call that a bad fit to the data. String theorists call it “qualitatively correct” which seems to mean their prediction has an upward slope.

But heavy ion physics is a messy business where many different processes come together and that makes it difficult to draw unambiguous conclusions. Clearly that situation doesn’t look good. However, as I mentioned earlier, last year I heard a talk by Steven Gubser about an upcoming paper of his addressing the energy loss in the gauge-gravity correspondence. Ficnar, Gubser and Gyulassy now recently posted their new paper on the arxiv:

In this paper, the authors propose a new description on the gravity side for the particle which on the field theory side loses energy while traveling through the plasma. Previously, this particle was modeled by a string parallel to the boundary that fell towards the black hole horizon. Ficnar et al instead model the particle by a string that ‘shoots up’ away from the horizon. They calculate the energy loss of the endpoint and find that the energy loss is reduced relative to the previous scenario.

They do not motivate the gravitational description and I am left wondering if not there should be an unambiguous procedure to find the gravitational analogue. If one can just choose a different setup and get a different energy loss that does not exactly increase my faith in the predictive value of the model.

Be that as it may, with their new model a sufficient reduction of the energy loss can only be achieved by pushing the crucial parameter (λ, the ‘t Hooft coupling) into a limit where the approximation actually breaks down. This is no good because then the results cannot be trusted.

So then they add higher curvature terms on the gravity side. This introduces an additional parameter, and a suitable choice for this second parameter allows the coupling to remain just about in the okay range. One would expect these higher-order terms to be present, but in principle I’d think the coupling shouldn’t be an independent parameter. In any case, this still doesn’t fit both the RHIC and the LHC data.

Since the interpretation of the data depends on the reconstruction of the effective temperature at the collision, they then speculate that maybe the temperature values are off by 10% or so, in which case their calculation would fit the data just fine.

This model is clearly an improvement though I can’t say I am terribly convinced. What seems to become increasingly clear though is that any successful model for highly energetic heavy ion collisions must use a suitable combination of both weakly and strongly coupled physics. The gauge-gravity correspondence still has a good chance to prove its use for the strongly coupled physics, but that will necessitate getting into all the messy details.

29 comments:

A few months ago I saw a talk by a researcher who came to our university (I do not remember his name, unfortunately) who described Ads-CFT duality to us. Now this isn't my field, but it seemed, based on his explanation that Ads-CFT was based on renormalization group theory and wasn't necessarily stringy.

Is there something in the results you talk about that directly relates them to string theory and not a more general mathematical technique, or am I misunderstanding gauge - gravity duality?

/* Ads-CFT was based on renormalization group theory and wasn't necessarily stringy */

It's because the AdS/CFT correspondence can be applied to many gauge theories, not just string theory. The application of holography to stringy model is similar to the application of AdS/CFT correspondence - it just narrows the parameter space while it makes model more specific at the price. The holography gets broken with extradimensions in similar way, like the AdS/CFT itself.

The future is in fully emergent one-dimensional string theory - no matter if you would call it the aether or in some more coward/opportunist way.

Zephir,a unit volume 1/philosophy x 1 x philosophy divided into (a+b+c)^3 parts always leaves a little replacation of itself to fill 3D with that important 4D ratio. Just like the divison of grain by the Eye of Horus the small error said given to the gods. What size then are the gods and how could they emerge in an endless fractal linear universe?You assert something wrong with the big picture as if this the solution to the issue. Just sayin '.

The string theory is physically substantiated from dense aether model perspective - well, just a bit. This is how the dense gas (condensing supercritical fluid) appears - it's density fluctuations resemble the strings.

The strings are emergent density fluctuations of hypothetical dense particle environment. If we would live at the water surface like the waterstriders and if we would observe it with its own ripples (as we do in vacuum), then the foamy density fluctuations would be the smallest and largest observable reality for you. Every particle would looks like foamy cluster of strings. And the space-time would appear like the foamy clusters of strings at the largest observable scale.

But from this analogy it's evident, the actual scope of string theory is limited to the extremely small and large distance scales. Anything in real life does appear like the quantum strings. We can only observe the traces of stringy model at the places, where the first density fluctuations of dense particle environment emerge: inside of atom nuclei, during particle collisions, inside of boson condensates, etc. At the moment, when these density fluctuations condense just a bit more, then the behavior of string theory model is lost again. Therefore the string theory applies just to the thin low-dimensional dimensional slices of hyperdimensional reality at the whole boundary of the observable universe. All just a bit more complex physical systems should be modeled with particle collisions from scratch. Why? Because these most subtle density fluctuations do appear like random unparticle blobs rather than one-dimensional strings. You can model the formation of strings with (density fluctuations of) pin-point particles, but not vice-versa.

One of properties of emergent fluctuations of particle environment is, they do appear similarly at the largest and smallest distance scale (as I noted already). At the water surface the smallest ripples are scattered with density fluctuations of underwater into the solitons in the geometrically similar way, like the largest circular ripples. The geometry of this scattering can be described with Lie groups and we can see its traces in dodecahedral geometry of dark matter fluctuations and structure of Higgs boson field. This is what the AdS/CFT duality is about. At the smaller and larger scales we can see the traces of this duality too: some planetary nebulae (composed of mostly atoms) resemble the atom orbitals, the dense neutron stars are similar to giant atom nuclei, etc. But again, this similarity is quite poorly expressed: you should know very well, where to look for it.

This is because the AdS/CFT duality is a low-dimensional theorem and the density fluctuations are becoming hyperdimensional fast in the same way, like the stringy fluctuations merge into spherical droplets during condensation. So that these models have very low application scope - they're merely a geometric curiosities of zero practical value.

Sabine, this is a bizarre and obviously vacuous way to criticize string theory. The point of science isn't to get something useful, its to discover how the universe works. The Higgs boson and Higgs mechanism are also an example of something "not useful", and quite appropriately, nobody cares because thats obviously not the point.

If you mean "useful" to for some reason include "part of the established scientific picture of how the world works" for some reason, then okay the Higgs boson would now be an example of something "useful", but it also would have been in the status of "struggling to demonstrate its use for anything at all" for all those decades before the experimental discovery. Is that supposed to mean all the theorists involved should have hung their heads in shame for those years for doing something so devoid of proven usefulness?

This is obviously a ridiculous standard to adopt. String theory's purpose – and use – has been known for some time. It has demonstrated itself to be a framework that consistently unifies all the fundamental principles that have been experimentally established to be part of this universe. And that fact has only become more significant as the years go by with no other idea having demonstrated those same necessary properties.

So please don't mislead the children with these snide comments. The purpose of string theory is clear and its "use" has been amply demonstrated.

Cliff, this is a bizarre and obviously vacuous way to criticize me. This blogpost is about the use of the AdS/CFT correspondence to explain recent data in heavy ion collisions. I think I have been extremely clear on this, so pls vent your anger elsewhere.

String theorists describe the physics of black holes in five-dimensional spacetime. They found that these five-dimensional objects provide a good approximation of the quark-gluon plasma in one fewer dimension, a relationship similar to the one between a three-dimensional object and its two-dimensional shadow. Image: SLAC National Accelerator Laboratory

The very foundation with which all attempts have been used, are all approximates, and you understand perfectly why.

TWO UNIVERSES of different dimension and obeying disparate physical laws are rendered completely equivalent by the holographic principle. Theorists have demonstrated this principle mathematically for a specific type of five-dimensional spacetime ("anti–de Sitter") and its four-dimensional boundary.

Consider any physical system, made of anything at all- let us call it, The Thing

You have to remember how you got here to the QGP, and the question of. So which of the examples would you choose to abandon?

In turn, experimental validations of these approximations could show how string theory best represents nature and point theorists toward avenues to explore further, Weidemann said. “But for now, string theory is just a useful tool for solving a current theory of reality.” See: String theory may hold answers about quark-gluon plasma Hmmm that's two years ago.....today?

It is much harder to play 2D chess in 3D than ND chess in 2D. You can start from either direction so imagine the other modeling as saying nothing useful. Quantization of space and time will be shown a given in both the math and physical phenomena.Did not David use a sling to slay a giant then compose the first songs about higher things on his strings?What good is the jawbone of an ass save to use as a weapon?Perhaps a review of some elementary geometry and arithmetic is in order here and the strength of intellect to consider better and higher models. The universe for now is much wiser than we seem to be. To find and recognize a higher unity of our models we should stop insisting on false distinctions and go about the business of addressing the keys to cosmic code order. And that if you really desire to know. We may hear the voices of prophets (since metaphysics seems to be the thing debaters here) but as to predictions - even Christ had to learn carpentry from Joseph.

“String theory, once hailed as theory of everything, now struggles to demonstrate its use for anything at all.Most string theorists today, if not working for banks, study the gauge-gravity correspondence.”

Come on Sabine, this is not fair and you know it; so why you are doing it?

Anyway QGP is just one aspect of AdS/CFT. In fact AdS/CFT is a theory of QG in the sense that it requires an UV complete theory of QG in the bulk (i.e. the full String theory not just its SUGRA limit) and CFT on the boundary which is also UV complete. It’s another definition of the full String theory if you like.

More to the point, I genuinely don't understand what you (and Cliff) have issues with. This is one in a series of posts in which I explain why the AdS/CFT correspondence is interesting and what's happening in that research area. Why are you unhappy with this? Would you rather see me reporting on recent research in Loop Quantum Gravity?

It is my impression that in fact most string theorists, if they're still around, work on AdS/CFT. Do you think this is incorrect? I also have the impression that string theory, now more than ever, is very under pressure to deliver, but not much is being delivered. That's what I wrote. I don't know what you think is "not fair" about this. Best,

I really appreciate your taking the time to blog about our paper. I am a big believer in pushing string theory toward meaningful contact with experiment. Without that push, can we call string theory part of physics?

Our least well justified step is to assert that energetic hard probes should be represented by the particular classical string motion in AdS that we study. Other aspects of our construction are, I think, more obviously within the range of accepted holographic phenomenology (with all the positives and negatives of this approach). For example, higher derivative terms are almost certainly part of the story in theories that come closer to QCD than N=4 SYM, and they provide a way to adjust viscosity upward into the range suspected to describe the QGP at LHC. The 10% adjustment in temperature could become a problem as studies of LHC heavy ion collisions become more refined; but it's generally good to remember that any hard probes calculation depends crucially on a lot of bulk dynamics, and 10% accuracy on temperature seems like a reasonable level of precision, at least for now.

At the end of the day, part of making direct, quantitative calculations with tools from string theory is that it's possible to be wrong---by 10%, by a factor of 2, or more. It's an indicator of how impressive the field has become that you don't "win" unless you do rather better than a factor of 2!

Thanks for chiming in. I'll follow the further developments with interest. I have no problem with the higher derivative terms per se, I was just wondering why the coupling is an independent parameter? Best,

Because we don't know the true gravity dual of QCD (assuming one exists), we instead start with a gravitational theory that we can embed in N=4 SYM (because that's solid ground) and then add the simplest term that can change the shear viscosity. I would not claim that this is a wholly systematic approach. Perhaps better would be to survey the effects of a variety of higher derivative terms on the type of calculation we are looking at.